1. The Field of the Invention
This invention relates to photopolymerizable compositions from which printing plates, or molds for making printing plates, are formed. This invention is particularly directed to printing plate molds having improved release characteristics.
2. The State of the Art
Photopolymerizable or photocurable polymers are used in the manufacture of printing plates. Such compositions typically contain monomers or "prepolymers" in which polymerization is initiated by light (actinic radiation). Polymerization can be initiated by using high energy radiation, such as in the ultraviolet frequencies, and the efficiency of the radiation is typically enhanced by the addition of a photoinitiator to the composition. The monomers or prepolymers include unsaturated bonds which participate in free radical polymerization upon exposure to actinic radiation, and thereby change the macroscopic structure of the composition from a fluid form to a solid form.
These photocurable resins can be used directly in the manufacture of printing plates. Typically, a liquid photocurable composition is cast onto a substrate, covered with a protective film, and selectively exposed to radiation in a desired pattern for reproduction; exposure is typically performed through a mask or transparency having opaque portions. Those portions of the composition exposed to actinic radiation polymerize and become insoluble to a solvent later used to wash away the unexposed portions. The entire plate is then exposed to radiation to assure that the composition over the entire surface of the plate is cured, which results in a printing plate devoid of tackiness. The result is a hardened photopolymer for use as a printing plate for imprinting the exposed pattern. The final plate is mounted onto a press; the relief portions of the printing plate pick up ink and so the plate can be used to make inked impressions.
U.S. Pat. No. 2,760,863 to Plambeck is related to such compositions. By the method in the Plambeck patent, printing plates are produced directly by exposing to actinic light, through an image bearing transparency, a layer of an essentially transparent photopolymerizable composition; this composition includes an addition polymerization initiator, and cross-links upon exposure to actinic radiation. The layer of photopolymerizable composition (suitably supported) is exposed until substantial polymerization of the composition occurs in the exposed areas; substantially no polymerization should occur in the unexposed, shaded areas. The unpolymerized material in the latter areas is then removed using a suitable solvent. The result is a raised relief image corresponding to the transparent areas in the transparency.
Compositions of the type described by Plambeck generally are prepared from polymeric compositions which require organic solvents for removal of the unexposed portions. Due to the toxicity, high volatility, and generally low flash point of low cost organic solvents, their use often gave rise to hazardous conditions. As a result, further research was directed towards the development of photopolymerizable compositions soluble in water or aqueous alkali. For example, U.S. Pat. Nos. 3,794,494 and 3,960,572, the disclosures of which are both incorporated herein by reference, describe water or aqueous alkali dispersible compositions which are suitable when polymerized for use as rigid or elastic flexographic printing plates. These patents describe the use of unsaturated polyesters or polyester-polyether urethane-linked block polymers containing acrylate or methacrylate end groups as the photopolymerizable composition. U.S. Pat. No. 4,198,238, the disclosure of which is incorporated herein by reference, describes aqueous-based photocurable compositions for fabricating soft printing plates suitable for printing with ultraviolet-curable inks; these compositions generally include a mixture of a polyether or polyester linked by a carbamoyl (urethane) to a terminal olefinic group in combination with a low molecular weight unsatured polyester and an ethylenically unsaturated monomer. Thus, a number of aqueous-based photocurable compositions are presently used; these systems allow the unexposed composition to be washed away with soap and water (i.e., water and suitable surfactants and/or detergents). The production of printing plates from a mixture of a photopolymerizable urethane prepolymer and an ethylenically unsaturated compound (such as vinylpyridine or acrylic acid) are described in U.S. Pat. No. 4,358,354, the disclosure of which is incorporated herein by reference. Further descriptions of photopolymer compositions for printing and other applications are contained in the following U.S. Pat. Nos. 3,989,609; 4,218,294; 4,218,299; 4,442,302; 4,837,126, and 4,849,321; the disclosures of which are all incorporated herein by reference.
The aforementioned U.S. Pat. No. 4,218,294 discloses photocurable coatings which, when used as a release coating or oxygen barrier, include the addition of a waxy material such as paraffin wax, ester wax (e.g., stearyl acetate, a saturated C.sub.18 fatty acid ester), fluorocarbon wax, or a silicon-containing material in amounts of 0.1% to 1.0% by weight.
As previously mentioned, these composition may also be useful in the fabrication of molds from which several printing plates can be cast. In this application, the photocurable composition is used to make a positive mold, which is then used to make a negative mold, and the negative mold is used to make a rubber positive printing relief. Generally, these photopolymerizable compositions are the same as those wherein the photopolymer is used directly for printing. Printing plates are relatively softer to facilitate transfer of printing ink to the substrate; in contrast, molds are subject to heat and pressure, and so rigidity is required to prevent distortion of the mold image. Accordingly, the use of relatively harder prepolymers and monomers with more rigid (sterically hindered) side chains, and of a relatively greater amount of cross-linking agents, are some means for tailoring the composition to mold making.
U.S. Pat. No. 4,749,750 discloses lubricating or release compounds for poly(vinyl chloride) (PVC) plastics which comprise a polyester with a comblike structure; the polyester is obtained by condensing at least one C.sub.12-14 hydroxy carboxylic acid with at least one monocarboxylic acid and/or glycerol chain terminator. These polyesters are described as providing superior lubrication properties in comparison with montan esters, i.e., esters from long-chain carboxylic acids with dihydric or polyhydric alcohols (e.g., glycol or glycerol). The release compounds are present in an amount of 0.3 to 3, preferably 0.5 to 1.0, parts by weight for each 100 parts of the PVC polymer. Montan wax is a hard, brittle mineral wax usually derived from lignites and is used especially for polishes, carbon paper, and insulating compositions.